Virtual private line communication is a service offered by some communication carriers that provides features and functionality similar to the well-known service of dedicated Private Line Automatic Ring-down (PLAR) between two end points. The virtual private line service (also known as "virtual hot line") was designed to replace, cost effectively, the expensive PLAR service that establishes a permanent point-to-point physical connection between two communication devices. Accordingly, the major features of PLAR, such as very short call setup time and secure communication between a calling and a called party, are also offered in the virtual private line service. Unlike PLAR, a virtual private line uses, as part of a communication switching system, a common software defined network to connect via trunks both ends of a circuit for the purpose of allowing an automatic connection to be established for the duration of a call, upon origination from either end. Subscribers of the virtual private line service, such as security traders in the financial industry, governmental security agencies and the armed forces attach a very high degree of importance to the "almost instantaneous connection" requirement of the service. To that end, most carriers have implemented a design wherein the mere lifting of a station's handset by the calling party automatically and virtually instantaneously results in a ringing tone at the called party's telephone set.
Equally important to virtual hot line subscribers is the requirement of guaranteed access to the party at the other end of the line. More specifically, subscribers of this service want the phone at one end of the line to always ring without any possibility of a busy tone upon lifting of the handset by a caller at the other end of the line. Dedicated private lines, by virtue of the permanent physical connection between the two communicating devices, can guarantee their subscribers access to parties at either end of the line. Aiming to meet this requirement for virtual private lines, in U.S. Pat. No. 4,982,421 issued on Oct. 27, 1989 to Kirsch et al. the inventors of that patent disclose a system for virtual private lines that uses non-dialable routing numbers devised by a software defined network to establish a connection between the two ends of the line. The use of the non-dialable routing number technique allows each end of the virtual private line connected to an access/egress switch to reach and to be reached only by the other end of the circuit because other potential calling parties have no way of activating the non-dialable routing number. Thus, the Kirsch et al. system partially meets the guaranteed access requirement of virtual private line subscribers by preventing inadvertent and unwanted calls from external parties who are not connected to the virtual private line from blocking access to the circuit. However, neither the Kirsch et al. system nor any other system in the prior art can prevent the software defined network from emitting busy signals in the instance of a "simultaneous" trunk seizure condition that occurs when both stations of the virtual private line try to establish a connection simultaneously (i.e. within the range of call setup time). This deficiency, which is called "glare", takes special significance when one considers that higher-than-usual incidences of busy signals can be reasonably expected for virtual private line service, since both parties would tend to initiate calls using the hot line within the same time window upon occurrence of special events requiring immediate communication between the parties.